The development of mouse CNS is being studied by light and electron microscopic analysis of organotypic cultures, prepared from normal brains and from brains with neurological mutant diseases. Myelin forms normally in cultures of cerebellum. Correlation of Nomarski optical studies of living cultures with scanning and transmission E. M. of the same cultures after fixation will permit delineation of the dynamic relationship between oligodendrocyte and axon during myelination. Two neurological mutants of the mouse, jimpy and quaking, have defective CNS myelination, probably brought about by different mechanisms. Both diseases are reproduced in culture: myelination is defective, other aspects of development are normal. Extension of correlative Nomarski - E. M. studies to mutant cultures may reveal a morphological abnormality underlying the failure of myelination. Synaptic networks also develop in culture: the hodology is partly known for cultured cerebellum but not for cultured hippocampus. Golgi and silver impregnations will be correlated with transmission E. M. to extend this knowledge, which may then be applied to cultures from brains with abnormalities of these structures such as staggerer, weaver and reeler.